--export bgen chrX/chrY bugfix, semver

This fixes a bug in --export bgen-1.{2,3}'s behavior on chrX and chrY, where data for the last few samples might not be properly exported (possibly resulting in an invalid .bgen).

In addition:

• "requires an alpha 6 or later build" error messages are now printed when appropriate.
• The version string has been reformatted to adhere to the Semantic Versioning 2.0.0 spec.
• The --lax-bgen-import flag has been backported.